Tapered roller bearings and gear shaft support devices

ABSTRACT

A tapered roller bearing and an automotive gear shaft support device can ensure a long endurance life even in the state in which debris is mixed. On the surfaces of an outer ring, inner ring, and tapered rollers formed from carburized bearing steel having an oxygen content of 9 ppm or less, carbo-nitrided layers having a carbon content of 0.80 wt % or over, a Rockwell hardness HRC of 58 or over, and a residual austenite content of 25-35 vol % are formed to increase mechanical properties and fatigue characteristics of the parts and to stably maintain the carbo-nitrided layers on the surfaces of the parts to a quality having suitable toughness, thereby markedly improving the endurance life of the tapered roller bearing in a state in which debris is mixed.

[0001] This application is a divisional application of application Ser.No. 10/200,777, filed Jul. 24, 2002, which is a divisional applicationof application Ser. No. 09/886,378, filed Jun. 22, 2001, now U.S. Pat.No. 6,447,168, which is a divisional application of application Ser. No.09/448,941, filed Nov. 24, 1999, now U.S. Pat. No. 6,328,477.

BACKGROUND OF THE INVENTION

[0002] This invention relates to tapered roller bearings and gear shaftsupport devices for vehicles.

[0003] Tapered roller bearings are suitable to support radial load,axial load and combined load. Because of their large load capacity, theyare used to support gear shafts of power transmission devices such asdifferentials and transmissions in automobiles and constructionmachines.

[0004]FIG. 1 shows an automotive differential in which a gear shaft issupported by tapered roller bearings which is one of the embodiments ofthe present invention. It basically comprises a drive pinion 4 rotatablysupported in a housing 1 by two tapered roller bearings 2, 3, a ringgear 5 meshing with the drive pinion 4, a differential gear case 7carrying the ring gear 5 and rotatably supported in the housing 1 by apair of tapered roller bearings 6, pinions 8 mounted in the differentialgear case 7, and a pair of side gears 9 meshing with the pinions 8.These members are mounted in the housing 1 in which is sealed gear oil.The gear oil also serves as a lubricating oil for the tapered rollerbearings 2, 3, 6.

[0005]FIG. 10 shows one conventional type of tapered roller bearing. Itcomprises an outer ring 52 having a conical raceway 51, an inner ring 56having a conical raceway 53, a large rib surface 54 on thelarge-diameter side of the raceway 53 and a small rib surface 55 on itssmall-diameter side, a plurality of tapered rollers 57 rollably arrangedbetween the raceway 51 of the outer ring 52 and the raceway 53 of theinner ring 56, and a retainer 58 keeping the tapered rollers 57circumferentially spaced a predetermined distance from each other. Thedistance between the large rib surface 54 and the small rib surface 55of the inner ring is designed to be slightly longer than the length ofthe tapered rollers 57.

[0006] The tapered rollers 57 are designed to come into line contactwith the raceways 51 and 53 of the outer ring 52 and the inner ring 56with the cone apexes of the tapered rollers 57 and the raceways 51, 53converging on a point O on the centerline of the tapered roller bearing.By this arrangement, the tapered rollers 57 can roll along the raceways51, 53.

[0007] With such a tapered roller bearing, the raceways 51, 53 havedifferent cone angles, so that the combined force of loads applied tothe tapered rollers 57 from the raceways 51, 53 acts in such a directionas to push the tapered rollers 57 toward the large rib surface 54 of theinner ring 56. Thus, during use of the bearing, the tapered rollers 57are guided with their large end faces 59 pressed against the large ribsurface 54, so that the large end faces 59 and the large rib surface 54are in sliding contact with each other.

[0008] On the other hand, since the distance between the large ribsurface 54 and the small rib surface 55 is designed to be slightlylonger than the length of the tapered rollers 57, as shown enlarged inFIG. 11, the small rib surface 55 does not contact the small end faces60 of the tapered rollers 57 such that small clearances existtherebetween. Also, the small rib surface 55 is formed by a surfaceinclined outwardly relative to the small end faces 60 of the taperedrollers 57. In the bearing manufacturing steps, the small rib surface 55and the small end faces 60, which are kept out of contact with eachother, are not finished by grinding.

[0009] In mounting such a tapered roller bearing in a mounting position,as shown in FIG. 12A, the assembly comprising the inner ring 56, thetapered rollers 57 and the retainer 58 is inserted into the raceway 51of the outer ring 52 from above with the large end faces 59 of thetapered rollers 57 facing up. At this time, since the tapered rollers 57have freedom relative to the inner ring 56 and the retainer 58, theywill not seat in position, and their small end faces 60 are brought intocontact with the small rib surface 55. This is an initial assembledstate in which clearance δ is present between the large end faces 59 andthe large rib surface 54 of the inner ring 56.

[0010] Next, the tapered roller bearing in the initial assembled stateis temporarily mounted on a mounting position of a mating device. Asshown in FIG. 12B, when break-in is carried out at a low speed of about50-100 rpm while applying an axial load Fa to the end face of the innerring 56, the tapered rollers 57 will move a distance equal to the gap δtoward the large rib surface 54, until as shown in FIG. 12C, the largeend faces 59 come into contact with the large rib surface 54 of theinner ring 56, so that they settle at a regular position during use ofthe bearing where a gap δ exists between the small end face 60 and thesmall rib surface 55.

[0011] Thereafter, the tapered roller bearing is preloaded axially undera predetermined load. This preloading is carried out to prevent axialmovement of the tapered rollers 57 during use of the bearing, and tostably bring the tapered rollers into line contact with the raceways 51,53 of the outer ring 52 and the inner ring 56. The control of preloadingforce is carried out by measuring the shaft torque, and preloading endswhen the shaft torque reaches a predetermined value.

[0012] Since the power transmission device such as a differential hasmany gear meshing portions and sliding portions of rotary members,foreign matter such as metallic powder produced by wear at theseportions can enter gear oil sealed in the housing. Such powder willpenetrate into tapered roller bearings for supporting gear shafts, whichare rotating under high load, thus shortening the working life of thetapered roller bearings.

[0013] Also, when such tapered roller bearings are used to support gearshafts of a differential which rotates at high speed under high load,since the large end faces of the tapered rollers are brought intosliding contact with the large rib surface of the inner ring, torque dueto the slide contact increases. Further, due to frictional heat buildup,the temperature of the bearing portion will rise, thus lowering theviscosity of gear oil. This may cause shortage of oil film.

[0014] Further, in mounting the tapered roller bearing on a mountingportion, if the gap between the large end faces 59 of the taperedrollers 57 and the large rib surface 54 is large in the initialassembled state shown in FIG. 12A, break-in time tends to be long untilthe tapered rollers 57 settle in their regular positions shown in FIG.12C. As shown in FIG. 11, since the small rib surface 55 of the innerring 56 is formed inclined outwardly relative to the the small end faces60 of the tapered rollers 57, variation in the gap between the large endfaces 59 and the large rib surface 54 in the initial assembled state islarge for the following reasons, and the abovementioned break-in timeuntil all the tapered rollers 57 settle in their regular positions tendsto become even longer.

[0015] Generally, the small end faces of the tapered rollers remain asforged surfaces, so that chamfer dimensions and shape are large invariation. Variations in chamfer dimension and shape are present notonly between tapered rollers but in a circumferential direction of onetapered roller. As shown by solid and chain lines in FIG. 11, if thechamfer dimension and shape of the small end faces 60 differ from eachother, the following will result. In the case of the small end faces 60shown by solid line, in the initial assembled state, point P1 on thesmall end face 60 comes into contact with point Q1 on the small ribsurface 55, so that the gap δ when the tapered rollers 57 settle will beδ₁. On the other hand, in the case of the small end face 60 shown bychain line, in the initial assembled state, point P2 comes into contactwith point Q2, so that the gap δ when the tapered rollers 57 settle willbe δ₂. Thus, due to differences in chamfer dimension and shape of thesmall end faces 60, the time until each tapered roller 57 settles inposition tends to vary, so that longer break-in time is required.

[0016] An object of this invention is to ensure a long endurance lifefor a tapered roller bearing and a gear shaft support device for avehicle.

[0017] Another object is to reduce torque loss and heat buildup due tofriction.

[0018] A further object is to shorten break-in time.

SUMMARY OF THE INVENTION

[0019] According to this invention, there is provided a tapered rollerbearing comprising an outer ring having a conical raceway, an inner ringhaving a conical raceway and formed with a large rib surface on thelarge diameter side of the conical raceway, a plurality of taperedrollers rollably arranged between the raceway of the outer ring and theraceway of the inner ring, and a retainer for keeping the taperedrollers circumferentially spaced a predetermined distance from eachother, characterized in that the outer ring, the inner ring and thetapered rollers are all formed from a steel having an oxygen content of9 ppm or less, and that a carbo-nitrided layer having a carbon contentof 0.80 wt % or more and a Rockwell hardness HRC of 58 or more is formedon surfaces of the outer ring, the inner ring and the tapered rollers,and that the retained austenite content of the carbo-nitrided layer is25 to 35 vol %.

[0020] The outer ring, inner ring and tapered rollers are formed from asteel having an oxygen content of 9 ppm or less in order to minimize anynonmetallic inclusions formed by oxides in the steel, improve themechanical characteristics and fatigue properties, and to sufficientlyensure bearing life under clean lubricating oil. A steel having anoxygen content of 9 ppm or less can be obtained e.g. by a method ofdegassing molten steel.

[0021] Carbo-nitrided layers are formed on the surfaces of the outerring, inner ring and tapered rollers for the following reasons. Retainedaustenite in a carburized layer obtained by normal carburizing has hightoughness and work hardening properties. Thus a proper amount ofretained austenite ensures hardness of the carburized layer andsuppresses initiation and progression of cracks. But it is unstableagainst heat.

[0022] In contrast, if these parts are subjected to carbo-nitridingtreatment under suitable conditions, nitrogen atoms will solid solutedin retained austenite, and thus serve to stabilize the-retainedaustenite against heat and also properly keep the properties of thecarbo-nitrided layer against a temperature rise due to temperature riseat the bearing portion. In a carbo-nitrided layer obtained by suchcarbo-nitriding treatment, a greater compressive residual stress isformed, so that it is also possible to further increase fatiguestrength.

[0023] The retained austenite content should be set at 25-35 vol % togive the carbo-nitrided layer proper toughness, and to relieve excessiveincrease in stress due to biting of debris. If the retained austenitecontent is less than 25 vol %, toughness would be insufficient. If over35 vol %, the hardness would be too low, thus resulting in deteriorationin surface roughness due to plastic deformation.

[0024] The structure of such a carbo-nitrided layer as mentioned abovecan be formed by the following treatment steps. After heating andholding the part for a predetermined time period while keeping thecarbon potential at 0.8% or over in a carburizing atmosphere, it isquenched in oil and is subjected to hardening. Thereafter it is heatedand held for a predetermined time period in ammonia gas for nitriding.It is also possible to employ a method in which nitriding is carried outduring carburizing. In order to adjust the retained austenite content,sub-zero treatment or tempering may be carried out.

[0025] According to this invention, a carbo-nitrided layer having acarbon content of 0.80 wt % or over and a Rockwell hardness HRC of 58 orover may be formed on the surfaces of the outer ring, inner ring andtapered rollers, the retained austenite amount of this carbo-nitridedlayer being 25 to 35 vol %, and crownings may be formed at both ends ofthe raceway of the inner ring, the width of the crowning at each endbeing 20% or less of the width of the raceway of the inner ring.

[0026] The crowning is formed at each end of the raceway of the innerring in order to prevent excessive edge loads from acting on the rollersand the raceway of the inner ring. The width of these crownings shouldbe 20% or less of the width of the raceway of the inner ring because ifit exceeds 20%, the contact surface pressure at the central portion ofthe raceway would be excessive.

[0027] By forming a crowning having a moderate curvature on a portion ofthe raceway of the inner ring except both ends at which the crowningsare formed, the surface pressure distribution on the raceway can be mademore uniform.

[0028] According to this invention, the small rib surface of the innerring may be formed by a surface parallel to the small end faces of thetapered rollers, the value R/R_(BASE) being 0.75 to 0.87, where R is theradius of curvature of the large end faces of the tapered rollers, andR_(BASE) is the distance from the apex of the cone angle of the taperedrollers to the large rib surface of the inner ring.

[0029] The small rib surface of the inner ring is formed by a surfaceparallel to the small end faces of the tapered rollers for the followingreasons. As shown enlarged in FIG. 6B, by forming the small rib surface34 of the inner ring 35 from a surface parallel to the small end faces39 of the tapered rollers 36, it is possible to minimize the influenceof variations in chamfer dimension and shape of the small end faces 39of the tapered rollers 36 against the gap between the large end faces 38of the tapered rollers 36 and the large rib surface 33 of the inner ring35 in the initial assembled state (which is equal to the gap between thesmall end faces 39 of the tapered rollers 36 and the small rib surfaces34 of the inner ring 35 when the tapered rollers 36 have settled inposition). As shown by chain line in FIG. 6B, even if the chamferdimensions and shapes of the small end faces 39 differ, in the initialassembled state, since the mutually parallel small end faces 39 andsmall rib surface 34 are brought into surface contact, the gap betweenthe large end faces 38 and the large rib surface 33 is always constant.Thus it is possible to reduce differences in time required until eachtapered roller settles and thus to shorten the break-in time.

[0030] The ratio of the radius of curvature R of the large end faces ofthe tapered rollers to the distance R_(base) from the apex of the coneangle of the tapered rollers to the large rib surface of the inner ring,R/R_(base) should be set at 0.75 to 0.87 for the following reasons.

[0031]FIG. 7 shows the results of calculation using the Karna'sequation, where t is the thickness of oil film formed between the largerib surface of the inner ring and the large end faces of the taperedrollers. The ordinate shows the ratio t/to, which is the ratio to oilfilm thickness to when R/R_(base)=0.76. The oil film thickness t is themaximum when R/R_(base)=0.76, and decreases sharply when R/R_(BASE)exceeds 0.9.

[0032]FIG. 8 shows the results of calculation for determining themaximum hertz stress p between the large rib surface of the inner ringand the large end faces of the tapered rollers. The ordinate shows, likeFIG. 7, the ratio p/po, which is the ratio to maximum hertz stress powhen R/R_(base)=0.76. The maximum hertz stress p monotonously decreaseswith an increase in R/R_(base).

[0033] In order to reduce torque loss and heat buildup due to slidingfriction between the large rib surface of the inner ring and the largeend faces of the tapered rollers, it is desirable to increase the oilfilm thickness t and reduce the maximum hertz stress p. Based on thecalculation results of FIGS. 7 and 8 and the below-mentioned seizureresistance test results, the present inventors determined the suitablerange of R/R_(base) at 0.75-0.87. For conventional tapered rollerbearings, the R/R_(base) value is designed at a range of 0.90-0.97.

[0034] By forming the surface roughness Ra of the large rib surface ofthe inner ring in the range of 0.05-0.20 μm, the oil film thickness tbetween the large rib surface of inner ring and the large end faces ofthe tapered rollers, and the lubricating condition between thesesurfaces can be maintained in a proper state.

[0035] The surface roughness Ra should be 0.05 μm or over for thefollowing reasons. As shown in FIG. 12B, when the tapered roller bearingis mounted, break-in is carried out at a low speed of 50-100 rpm whileapplying an axial load Fa to the end face of the inner ring 56. If thesurface roughness Ra is less than 0.05 μm, the lubricating state betweenthe large rib surface 54 of the inner ring 56 and the large end faces 59of the tapered rollers 57 will involve a mixture of fluid lubricationand boundary lubrication during break-in, so that the frictioncoefficient varies considerably and the measured shaft torque varieswidely. This worsens the preload control accuracy. If Ra is 0.05 μm orover, the lubricating state will be boundary lubrication, so that thefriction coefficient stabilizes and thus preload control is possiblewith high accuracy. Under normal bearing use conditions where speedexceeds 100 rpm, sufficient oil film is formed between the large ribsurface 54 and the large end faces 59, so that the lubricating statebetween these surfaces becomes fluid lubrication, and the frictioncoefficient decreases.

[0036] The surface roughness Ra should be 0.20 μm or under because if Rais over 0.20 μm, the temperature will rise at the bearing portion in thehigh-speed rotation region, so that when the viscosity of lubricatingoil decreases, the oil film thickness tends to be insufficient andseizure tends to occur.

[0037] By restricting the gap δ formed between the small rib surface ofthe inner ring and the small end faces of the tapered rollers when thelarge end faces of the tapered rollers are in contact with the large ribsurface of the inner ring to not more than 0.4 mm, it is possible toreduce the number of revolutions required for the tapered rollers tosettle in position during the break-in, and to shorten the break-intime. The permissible maximum value of the gap δ, that is, 0.4 mm, wasdetermined based on the results of the below-described break-in test.

[0038] By forming the small rib surface of the inner ring by grinding orturning, it is possible to accurately control the gap between the smallrib surface of the inner ring and the small end faces of the taperedrollers.

[0039] The tapered roller bearing of this invention may have the largerib surface of the inner ring made up of a conical surface in contactwith the large end faces of the tapered rollers, and a flank smoothlyconnecting with the conical surface and curving in a direction away fromthe large end faces of the tapered rollers.

[0040] By smoothly connecting the curved flank to the conical surface ofthe large rib surface of inner ring in contact with the large end facesof the tapered rollers and forming an acute-angle, wedge-shaped gap nearthe outer edge of the contact region, it is possible to increase thefunction of drawing lubricating oil into the contact region and to forma good oil film. Also, by the formation of the smooth flank, it ispossible to prevent damage due to abutment with the large rib surface ofinner ring when the tapered roller skews.

[0041] By employing an arc as the sectional shape of the flank, it ispossible to easily form a flank that is superior in the lubricating oildrawing function.

[0042] By providing a circular recess on the central portion of thelarge end faces of the tapered rollers, and extending the outerperipheral end of the recess to near the boundary between the conicalsurface and the flank of the large rib surface of the inner ring, it ispossible to guide lubricating oil to near the wedge-shaped gap and tosupply a sufficient amount of lubricating oil into the wedge-shaped gap,and also to further increase the permissible skew angle of the taperedrollers.

[0043] By providing the boundary between the conical surface and theflank of the large rib surface of inner ring near the outer edge of themaximum contact oval produced by the contact between the large end facesof the tapered rollers and the large rib surface of the inner ring underthe maximum permissible axial load of the tapered roller bearing, it ispossible to suitably form the wedge-shaped gap for drawing thelubricating oil in the entire load range of the tapered roller bearing.

[0044] Also, in this invention, in a gear shaft support device for avehicle in which a gear shaft is rotatably supported by a tapered rollerbearing in a housing in which is sealed gear oil, the outer ring, innerring and tapered rollers of the tapered roller bearings are formed froma steel having an oxygen content of 9 ppm or less, and a carbo-nitridedlayer having a carbon content of 0.80 wt % or more and a Rockwellhardness HRC of 58 or more is formed on each of their surfaces, thecarbo-nitrided layer having a retained austenite amount of 25 to 35 vol%. Thus it is possible to markedly prolong the maintenance cycle ofdifferentials and transmissions, etc.

[0045] Other features and objects of the present invention will becomeapparent from the following description made with reference to theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0046]FIG. 1 is a vertical sectional view of a differential in which isassembled a gear shaft support device of a first embodiment;

[0047]FIG. 2A is a vertical sectional view of a tapered roller bearingof a first embodiment;

[0048]FIG. 2B is a partially enlarged sectional view of the same;

[0049]FIG. 3 is a partially enlarged sectional view of a tapered rollerbearing of a second embodiment;

[0050]FIG. 4A is a vertical sectional view of a tapered roller bearingof a third embodiment;

[0051]FIG. 4B is a partially enlarged sectional view of the same;

[0052]FIG. 5 is a sectional view explaining the design specifications ofthe tapered roller bearing of FIG. 4;

[0053]FIG. 6A is a vertical sectional view of a tapered roller bearingof a fourth embodiment;

[0054]FIG. 6B is a partially enlarged sectional view of the same;

[0055]FIG. 7 is a graph showing the relation between the radius ofcurvature of the large end face of the tapered roller and an oil filmthickness;

[0056]FIG. 8 is a graph showing the relation between the radius ofcurvature of the large end face of the tapered roller and a maximumhertz stress;

[0057]FIG. 9 is a partially enlarged sectional view of a tapered rollerbearing of a fifth embodiment;

[0058]FIG. 10 is a partially omitted vertical sectional view of aconventional tapered roller bearing;

[0059]FIG. 11 is a partially enlarged sectional view of FIG. 10; and

[0060]FIG. 12A-12C are sectional views showing how the tapered rollerbearing is mounted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] With reference to FIGS. 1 and 9, embodiments of this inventionare described. FIG. 1 shows, as described above, a differential of anautomobile, in which for the support of the drive pinion 4 and thedifferential gear case 7 on which is mounted the ring gear 5, the gearshaft support device using the tapered roller bearings 2, 3, 6 of theembodiments is adopted.

[0062]FIG. 2A shows a tapered roller bearing 6 as a typical example. Ithas an outer ring 11 having a conical raceway 10, an inner ring 15having a conical raceway 12, a large rib surface 13 on thelarge-diameter side of the raceway 12, and a small rib surface 14 on itssmall-diameter side, a plurality of tapered rollers 16 rollably arrangedbetween the respective raceways 10, 12 of the outer ring 11 and theinner ring 15, and a retainer 17 for retaining the tapered rollers 16 atpredetermined circumferential intervals.

[0063] The outer ring 11, inner ring 15 and tapered rollers 16 are allformed from carburized bearing steel (SCr 435) having an oxygen contentof 9 ppm or less, and as shown in FIG. 2B, carbo-nitrided layers 11 a,15 a, 16 a having a carbon content of 0.80 wt % or more and a Rockwellhardness HRC of 58 or more, and the retained austenite content of 25 to35 vol % are formed on the surfaces of these parts 11, 15 and 16. Thoughnot shown, the tapered roller bearings 2, 3 have the same structure.

[0064] Hereinbelow, the Examples of the first embodiment and itsComparative Examples are described.

EXAMPLES

[0065] Tapered roller bearings (Examples 11-15 in Table 1) in which acarbo-nitrided layer having a carbon content of 0.80 wt % or more and aRockwell hardness HRC of 58 or more and the retained austenite contentof 25-35 vol % was formed on each of the outer ring, inner ring andtapered rollers formed from carburized bearing steel (SCr435) having anoxygen content of 9 ppm or less were prepared. The bearing dimensionswere all 40 mm in inner diameter and 68 mm in outer diameter.

Comparative Examples

[0066] Tapered roller bearings (Comparative Examples 11-15 in Table 1)in which, similar to the Examples, a carbo-nitrided layer having acarbon content of 0.80 wt % or over and a Rockwell hardness HRC of 58 orover and the retained austenite content of 25-35 vol % was formed oneach of the outer ring, inner ring and tapered rollers formed fromcarburized bearing steel (SCr435) having an oxygen content exceeding 9ppm, and tapered roller bearings (Comparative Examples 16, 17 inTable 1) in which the outer ring, inner ring and tapered rollers wereformed from carburized bearing steel (SCr435) having an oxygen contentof 9 ppm or less but the carbo-nitrided layer formed thereon had aretained austenite content outside the range as claimed in the presentinvention were prepared. Also, a tapered roller bearing (ComparativeExample 18 in Table 1) in which carburized bearing steel (SCr435) havingan oxygen content exceeding 9 ppm was used and heat treatment with onlyordinary carburizing was prepared. The dimensions of each bearing werethe same as in Examples of the invention.

[0067] A debris contamination life test in which the tapered rollerbearings of the Examples of the invention and Comparative Examples weremounted on a rotary shaft arranged in a case in which was sealed alubricating oil in which was mixed debris, and a clean oil life test inwhich they were mounted on a rotary shaft arranged in a case in whichclean lubricating oil was circulated were conducted.

[0068] The test conditions are as shown below.

[0069] (Debris Contamination Life Test)

[0070] Load: 11.76 kN

[0071] Revolutional speed: 1500 rpm

[0072] Lubricating oil: turbine oil VG56 (oil bath)

[0073] Debris: gas atomized metallic powder (particle diameter: 100-180μm, hardness: HV 700-800, mixed amount: 1 g/liter)

[0074] (Clean Oil Life Test)

[0075] Load: 21.56 kN

[0076] Revolutional speed: 2000 rpm

[0077] Lubricating oil: turbine oil VG 56 (circulation oil supply)

[0078] The test results are shown in Table 1. In the debriscontamination life test and the clean oil life test, the lives wereevaluated in terms of L10 life (time period during which 90% of thebearings were not destroyed and usable). Also, for the life ratios, theendurance life of the bearing of Comparative Example 18, which wasmanufactured under ordinary conditions both in material and heattreatment, was used as a reference value.

[0079] It is apparent that the tapered roller bearings of the Examplesshow excellent results both in the debris contamination life test andclean oil life test. On the other hand, Comparative Examples 11-15, inwhich the retained austenite content was in the range of 25-35 vol % butthe oxygen content of the steel was high, showed good results in thedebris contamination life test, but inferior results in the clean oillife test. Also, for Comparative Examples 16-17, in which the retainedaustenite content was out of the range of the present application, theendurance life in the clean oil life test was a relatively high value,but that of the debris contamination life test was inferior.

[0080]FIG. 3 shows in enlarged scale a portion of the tapered rollerbearing of the second embodiment. It has edge crownings 20 having awidth Wc which is 20% or less of the width W of the raceway 19, at bothends of the raceway 19 of the inner ring 18. At the central portionbetween these respective crownings 20, a center crowning 21 having anextremely moderate curvature is formed. The drop amount D of thecrownings 20 is 20 μm, and outside the crownings 20, recesses 22 areprovided.

[0081] This tapered roller bearing, too, is used to support adifferential gear case 7 like the one shown in FIG. 1, and each part isformed from carburized bearing steel (SCr 435), and like the taperedroller bearing 6 shown in FIG. 2, carbon-nitrided layers having a carboncontent of 0.8 wt % or over and a Rockwell hardness HRC of 58 or over,and the retained austenite content of 25-35 vol %, are formed on theirsurfaces.

[0082] Hereinbelow, the Examples of the second embodiment and itsComparative Examples are described.

EXAMPLES

[0083] Tapered roller bearings (Examples 21-25 in Table 2) in which acarbo-nitrided layer having a carbon content of 0.80 wt % or over, aRockwell hardness HRC of 58 or over and a retained austenite content of25-35 vol % was formed on each of the outer ring, inner ring and taperedrollers formed from carburized bearing steel (SCr435), and edgecrownings having a width Wc which was 20% or less of the width W of theinner ring raceway were formed at both ends of the raceway wereprepared. The tapered roller bearings of Examples 21 through 23 wereformed with a center crowning having a crowning amount C of 2 μm at thecenter of the inner ring raceway, while the tapered roller bearings ofExamples 24 and 25 were not. The bearing dimensions are the same as inthe first embodiment.

Comparative Examples

[0084] Tapered roller bearings (Comparative Examples 21-24 in Table 2)in which, similar to the Examples, a carbo-nitrided layer having acarbon content of 0.80 wt % or over and a Rockwell hardness HRC of 58 orover was formed on each of the outer ring, inner ring and taperedrollers formed from carburized bearing steel (SCr435), but the retainedaustenite content in the carbo-nitrided layers was out of the range asclaimed in the present application, and tapered roller bearings(Comparative Examples 25-27 in Table 2) in which the retained austenitecontent was within the range of the present application, but the widthof edge crownings exceeded the range of the present application, or fullcrowning was formed over the entire width of the inner ring raceway wereprepared. In Comparative Examples 22 and 24, the width of edge crowningalso exceeded the range of the present application. Also, a taperedroller bearing (Comparative Example 28 in Table 2) in which the retainedaustenite content and the width of edge crowning were within the rangeof the present application, and the heat treatment was ordinarycarburizing hardening was prepared. Dimensions of each bearing were thesame as in the Examples.

[0085] For the tapered roller bearings of the Examples and ComparativeExamples, a debris contamination life test was conducted. The testconditions were the same as those in the first embodiment, and theendurance life was evaluated in terms of L10 life.

[0086] The test results are shown in Table 2. For the life ratios in thetable, the endurance life of Comparative Example 28, in which the heattreatment was only carburizing hardening, was used as a reference value.For any of the articles so indicated in the Table, seizure occurred atthe central portion of the raceway.

[0087] For each of the tapered roller bearings of the Examples, the liferatio was more than four-fold and showed an excellent endurance life.Also, no seizure occurred at the central portion of the raceway. On theother hand, Comparative Examples 21-24, in which the retained austenitecontent was out of the range of the present application, had only abouthalf the life ratio of the tapered roller bearings of the Examples. ForComparative Examples 22 and 24, which were large in crowning width,seizure occurred at the central portion of the raceway. Also, forComparative Examples 25 and 26, in which the retained austenite contentwas in the range of the present application, but the crowning width waslarge, the life ratio was good, but seizure occurred at the central partof the raceway. For Comparative Example 27, which was extremely small indrop amount D, peeling occurred at the ends of the raceway, and the liferatio improved little.

[0088]FIGS. 4A and 4B show the third embodiment. This tapered rollerbearing was also used for the support of a differential gear case 7 likethe one shown in FIG. 1, and their parts, that is, the outer ring 23,inner ring 24 and tapered rollers 25, were formed from carburizedbearing steel (SCr435), and carbo-nitrided layers 23 a, 24 a, 25 ahaving a carbon content of 0.80 wt % or over and a Rockwell hardness HRCof 58 or over were formed on the surfaces of these parts as shown inFIG. 4B.

[0089] As shown in FIG. 5, the cone angle apex of the tapered rollers25, and the respective raceways 26, 27 of the outer ring 23 and innerring 24 converge at one point on the centerline of the tapered rollerbearing, and it is manufactured such that the ratio of the radius ofcurvature R of the large end faces 28 of the tapered rollers 25 to thedistance R_(base) from point O to the large rib surface 29 of the innerring 24, i.e. R/R_(base), is in the range of 0.75-0.87. Also, the largerib surface 29 is ground to the surface roughness Ra of 0.12 μm.

[0090] Hereinbelow, the Examples of the third embodiment and itsComparative Examples are described.

EXAMPLES

[0091] Tapered bearings (Examples 31-34 in Table 3) shown in FIGS. 4A,4B and 5, were prepared in which a carbo-nitrided layer having a carboncontent of 0.8 wt % or over and a Rockwell hardness HRC of 58 or overwas formed on the surface of each of the outer ring, inner ring andtapered rollers, which were formed from carburized bearing steel SCr435,in which the radius of curvature R of the large end faces of the taperedrollers was in the range of R/R_(base)=0.75 to 0.87, and in which thesurface roughness Ra of the large rib surface of the inner ring was 0.12μm. Dimensions of the bearings were the same as in the first and secondembodiments.

Comparative Examples

[0092] Tapered bearings (Comparative Examples 31-33 in Table 3) wereprepared in which, like the Examples, a carbo-nitrided layer having acarbon content of 0.8 wt % or over and a Rockwell hardness HRC of 58 orover was formed on the surface of each of the outer ring, inner ring andtapered rollers which were formed from carburized bearing steel SCr435,but the R/R_(base) ratio was out of the range of the presentapplication, and a tapered roller bearing (Comparative Example 34 inTable 4) in which the heat treatment was only carburized and hardening,and the R/R_(base) ratio was also out of the range of the presentapplication was prepared. Dimensions of the bearings are the same as inthe Examples.

[0093] For the Examples and Comparative Examples, a seizure resistancetest using a rotary tester, and the same debris contamination life testas in the first and second embodiments were conducted.

[0094] The test conditions of the seizure resistance test were asfollows.

[0095] Load: 19.61 kN

[0096] Revolutional speed: 1000-3500 rpm

[0097] Lubricating oil: turbine oil VG 56 (oil supply rate: 40mililiters/minute, oil temperature: 40° C.±3° C.)

[0098] The test results are shown in Table 3. For the life ratios in thedebris contamination life test, the endurance life (L10 life) ofComparative Example 34 was used as a reference value. Also, seizure inthe seizure resistance test occurred between the large rib surface ofthe inner ring and the large end faces of the tapered rollers.

[0099] For each of the tapered roller bearings of the Examples, theendurance life was good with the life ratio in the debris contaminationlife test being four or more. Also, it is apparent that the limitrevolving speed at which seizure occurred in the seizure resistance testwas 2700 rpm or over. On the other hand, for Comparative Examples 31-33,in which carbo-nitrided layers were formed, but the R/R_(base) ratio wasout of the range of the present application, although the life ratio wasgood, the limit revolving speed for the occurrence of seizure was 2500rpm or under, and the possibility that seizure may occur under normaluse conditions such as in a differential was high. For ComparativeExample 33, in which the surface roughness Ra of the large rib surfacewas rough, it showed a limit revolving speed that was lower than inComparative Example 32 having the same radius of curvature. ForComparative Example 34, in which heat treatment was ordinarycarburizing, and also R/R_(base) ratio was a conventional value, any ofthe test results were inferior.

[0100] In the above embodiments, SCr435 was used as a material for eachpart, but it is possible to use such bearing steels as SCM420, SCM430,SCM435, SCr420, SCr430, SAE5130, and SAE8620.

[0101]FIGS. 6A and 6B show the fourth embodiment. This tapered rollerbearing is also used for the support of a differential gear case 7 as inFIG. 1, and comprises an outer ring 31 having a conical raceway 30, aninner ring 35 having a conical raceway 32 and provided with a large ribsurface 33 on the large-diameter side of the raceway 32 and a small ribsurface 34 on its small-diameter side, a plurality of tapered rollers 36arranged between the respective raceways 30, 32 of the outer ring 31 andthe inner ring 35, and a retainer 37 retaining the tapered rollers 36 atpredetermined circumferential intervals.

[0102] The small rib surface 34 of the inner ring 35, as shown enlargedin FIG. 6B, is finished to a ground surface parallel to the small endfaces 39 of the tapered rollers 36 arranged on the raceway 32. It is insurface contact with the small end faces 39 of the tapered rollers 36 inthe initial assembled state shown by one-dot chain line in FIG. 6B, andthe gapδ with respect to the small end faces 39 of the tapered rollers36 in the state in which the tapered rollers 36 have settled in positionas shown by solid line is in the range of not more than 0.4 mm. Thesmall rib surface 34 may be finished by turning to reduce the cost.

[0103] The cone angle apexes of the tapered rollers 36, and therespective raceways 30, 32 of the outer ring 31 and inner ring 35converge, like the third embodiment shown in FIG. 5, at one point O onthe centerline of the tapered roller bearing, and it is manufacturedsuch that the ratio of the radius of curvature R of the large end faces38 of the tapered rollers 36 to the distance R_(BASE) from point O tothe large rib surface 33 of the inner ring 35, i.e. R/R_(base), is inthe range of 0.75-0.87. Also, the large rib surface 33 is ground to thesurface roughness Ra of 0.12 μm.

[0104] The Examples of the fourth embodiment and its ComparativeExamples are described below.

EXAMPLES

[0105] Tapered roller bearings (Examples 41-44 in Table 4) were preparedin which the radius of curvature R of the large end faces of the taperedrollers was such that the ratio R/R_(base) was 0.75-0.87, the surfaceroughness Ra of the large rib surface of the inner ring was 0.12 μm, itssmall rib surface was formed as a ground surface parallel to the smallend faces of the tapered rollers, and the gap δ was in the range of notmore than 0.4 mm. Bearing dimensions were the same as in each of theabovementioned embodiments.

Comparative Examples

[0106] Tapered roller bearings (Comparative Examples 41-43 in Table 4)were prepared in which the R/R_(base) value was out of the range of thepresent application, the small rib surface of the inner ring wasinclined outwardly relative to the small end faces of the taperedrollers, and the gap δ exceeded 0.4 mm.

[0107] For the tapered roller bearings of the Examples of andComparative Examples, a seizure resistance test was conducted under thesame conditions as in the third embodiment. Also, for the tapered rollerbearings of Example 42 and Comparative Example 42, a break-in test wasalso conducted. Sample numbers for the break-in test were 66 for Example42 and 10 for Comparative Example 42.

[0108] The results of the test are shown in Table 4. Seizure in theseizure resistance test occurred between the large rib surface of theinner ring and the large end faces of the tapered rollers.

[0109] For any of the tapered roller bearings of the Examples, the limitrevolving speed in the seizure resistance test was 2700 rpm or over.This shows that the frictional resistance between the large rib surfaceof the inner ring and the large end faces of the tapered rollers issmall. On the other hand, for the tapered roller bearings of theComparative Examples, the seizure occurrence limit revolving speed was2500 rpm or under, and a problem may arise under normal use conditionssuch as in a differential. For Comparative Example 43, in which thesurface roughness Ra of the large rib surface was rough, it showed alower seizure occurrence limit revolving speed than in ComparativeExample 42 having the same radius of curvature R.

[0110] For the break-in test results, in the Comparative Examples, theaverage value of the number of revolutions until the tapered rollerssettled in position was six, whereas in the Examples, this value wasabout half, i.e. 2.96. In the Examples of the invention, the standarddeviation of variation in the number of revolutions was also small.Thus, this shows that it is possible to stably shorten the break-intime.

[0111]FIG. 9 shows a portion of the tapered roller bearing of the fifthembodiment. This tapered roller bearing was also used for the support ofa differential gear case 7 as shown in FIG. 1. The large rib surface 41of the inner ring 40 comprises a conical surface 41 a, and a flank 41 bsmoothly connecting with the conical surface 41 and having an arcuatesection, and a chamfer 41 c connecting with the flank 41 b. The conicalsurface 41 a is, like the tapered roller bearing shown in FIG. 5, formedwith point O as its center. The end faces 43 of the tapered rollers 42are each formed as a spherical surface 43 a having a radius of curvatureR that is smaller than the distance Ro from point O to the large ribsurface 41 of the inner ring 40. A recess 44 of a circular shape isformed at the center of the spherical surface 43 a. The outer peripheralend of the recess 44 extends to near the boundary between the conicalsurface 41 a and the flank 41b of the large rib surface 41.

[0112] As mentioned above, during use of the bearing, the taperedrollers 42 roll with their large end faces 43 pressed against the largerib surface 41, and the spherical surface 43 a is partially brought intocontact with the conical surface 41 a, so that a contact oval 45 isproduced between these two curved surfaces. The boundary between theflank 41 b and the conical surface 41 a is provided near the outer edgeof the contact oval 45, and an acute wedge-shaped gap is defined by theflank 41 b and the spherical surface 43 a at a position near the contactoval 45.

[0113] The contact oval 45 grows larger as the axial load during use ofthe bearing increases. With this tapered roller bearing, assuming themaximum contact oval under the permissible maximum axial load, theboundary between the flank 41 b and the conical surface 41 a is designedto be near the outer edge of the maximum contact oval, so that thewedge-shaped gap for drawing the lubricating oil will be formed over theentire load range.

[0114] The present invention is applicable to various types of taperedroller bearings.

[0115] As described above, for the tapered roller bearing of thisinvention, each of its parts, i.e. outer ring, inner ring and taperedrollers are formed from steel having an oxygen content of 9 ppm or less,and a carbo-nitrided layer having a carbon content not less than 0.80 wt% and a Rockwell hardness HRC of 58 or over and the retained austenitecontent of 25-35 vol % is formed on the surface of these parts. Thus itis possible to enhance the mechanical properties and fatigue strength ofthe parts, stably maintain the carbo-nitrided layers on the surfaces ofthe parts to a quality having suitable toughness, and markedly improvethe endurance life in debris contamination conditions.

[0116] According to this invention, an edge crowning having a width thatis 20% or less of the width of the raceway is formed at both ends of theinner ring raceway. This prevents seizure by making the contact surfacepressure at the raceway uniform, maintains the carbo-nitrided layers onthe surfaces of the parts stably to a quality having suitable toughness,and markedly improves the endurance life in debris contaminationconditions.

[0117] According to this invention, the radius of curvature R of thelarge end faces of the tapered rollers is such that the ratio R/R_(base)will be 0.75-0.87, and the small rib surface of the inner ring is formedinto a surface parallel to the small end faces of the tapered rollers toprevent seizure by reducing torque loss and heat buildup due to slidingfriction between the large rib surface of the inner ring and the endfaces of the tapered rollers, and to shorten the break-in time toimprove efficiency of mounting of the bearing.

[0118] Further, according to this invention, a curved flank is smoothlyconnected to the conical surface of the large rib surface of the innerring in contact with the large end faces of the tapered rollers to forman acute wedge-shaped gap to increase the lubricating oil drawingfunction into this contact region, prevent seizure by reducing torqueloss and heat buildup due to the sliding friction, and prevent seizuredue to abutment with the large rib surface of the inner ring duringtapered roller skew.

[0119] With the gear shaft support device of this invention, since itsgear shaft is supported by the tapered roller bearing of this invention,endurance life improves even under use conditions in which foreignmatter mixes into gear oil, so that it is possible to extremely prolongthe maintenance cycle of a power transmission device such as adifferential.

What is claimed is:
 1. A tapered roller bearing comprising an outer ring having a conical raceway, an inner ring having a conical raceway and formed with a large rib surface on the large diameter side of said conical raceway, a plurality of tapered rollers rollably arranged between said raceway of said outer ring and said raceway of said inner ring, and a retainer for keeping said tapered rollers circumferentially spaced a predetermined distance from each other, characterized in that said outer ring, said inner ring and said tapered rollers are all formed from a steel having an oxygen content of 9 ppm or less, and that a carbo-nitrided layer having a carbon content of 0.80 wt % or more and a Rockwell hardness HRC of 58 or more is formed on surfaces of said outer ring, said inner ring and said tapered rollers, and that the retained austenite content of said carbo-nitrided layer is 25 to 35 vol %.
 2. A tapered roller bearing comprising an outer ring having a conical raceway, an inner ring having a conical raceway and formed with a large rib surface on the large diameter side of said conical raceway, a plurality of tapered rollers rollably arranged between said raceway of said outer ring and said raceway of said inner ring, and a retainer for keeping said tapered rollers circumferentially spaced a predetermined distance from each other, characterized in that a carbo-nitrided layer having a carbon content of 0.80 wt % or more and a Rockwell hardness HRC of 58 or more is formed on surfaces of said outer ring, said inner ring and said tapered rollers, that the retained austenite content of said carbo-nitrided layer is 25 to 35 vol %, and crownings are formed at both ends of said raceway of said inner ring, and that the width of each said crowning is 20% or less of the width of said raceway of said inner ring.
 3. The tapered roller bearing as claimed in claim 2 wherein a crowning having a moderate curvature is formed on a portion of said raceway of said inner ring except both ends thereof at which said crownings are formed.
 4. A tapered roller bearing comprising an outer ring having a conical raceway, an inner ring having a conical raceway and formed with a large rib surface on the large diameter side of said conical raceway, a plurality of tapered rollers rollably arranged between said raceway of said outer ring and said raceway of said inner ring, and a retainer for keeping said tapered rollers circumferentially spaced a predetermined distance from each other, characterized in that said inner ring has a large rib surface made up of a conical surface brought into contact with large end faces of said tapered rollers, and a flank smoothly connecting with said conical surface and curving in a direction away from the large end faces of said tapered rollers.
 5. The tapered roller bearing as claimed in claim 4 wherein said flank has a circular section.
 6. The tapered roller bearing as claimed in claim 5 wherein a circular recess is provided on the central portion of each of the large end faces of said tapered rollers, and the outer peripheral end of said recess extends to near the boundary between said conical surface and said flank of said large rib surface of said inner ring.
 7. The tapered roller bearing as claimed in claim 6 wherein the boundary between said conical surface and said flank of said large rib surface of said inner ring is provided near the outer edge of the maximum contact oval produced by the contact between the large end faces of said tapered rollers and the large rib surface of said inner ring under the maximum permissible axial load of said tapered roller bearing.
 8. The tapered roller bearing as claimed in claim 4 wherein a circular recess is provided on the central portion of each of the large end faces of said tapered rollers, and the outer peripheral end of said recess extends to near the boundary between said conical surface and said flank of said large rib surface of said inner ring.
 9. The tapered roller bearing as claimed in claim 8 wherein the boundary between said conical surface and said flank of said large rib surface of said inner ring is provided near the outer edge of the maximum contact oval produced by the contact between the large end faces of said tapered rollers and the large rib surface of said inner ring under the maximum permissible axial load of said tapered roller bearing.
 10. The tapered roller bearing as claimed in claim 4 wherein the boundary between said conical surface and said flank of said large rib surface of said inner ring is provided near the outer edge of the maximum contact oval produced by the contact between the large end faces of said tapered rollers and the large rib surface of said inner ring under the maximum permissible axial load of said tapered roller bearing.
 11. A gear shaft support device for a vehicle in which a gear shaft is rotatably supported by tapered roller bearings in a housing in which is sealed gear oil, characterized in that said tapered roller bearings each have an outer ring, an inner ring and tapered rollers formed from a steel having an oxygen content of 9 ppm or less, and that a carbo-nitrided layer having a carbon content of 0.80 wt % or more and a Rockwell hardness HRC of 58 or more is formed on surfaces of said inner ring, said outer ring and said tapered rollers, said carbo-nitrided layer having a retained austenite content of 25 to 35 vol %.
 12. A gear shaft support device for a vehicle in which a gear shaft is rotatably supported by tapered roller bearings in a housing in which is sealed gear oil, said tapered roller bearings each having an outer ring, an inner ring and tapered rollers, characterized in that a carbo-nitrided layer having a carbon content of 0.80 wt % or more and a Rockwell hardness HRC of 58 or more is formed on each of the surfaces of said outer ring, said inner ring and said tapered rollers, that said carbo-nitrided layer has retained austenite content of 25 to 35 vol %, and that crownings are formed at both ends of said raceway of said inner ring, the width of each said crowning being 20% or less of the width of said raceway of said inner ring.
 13. The gear shaft support device as claimed in claim 12 wherein a crowning having a moderate curvature is formed on a portion of said raceway of said inner ring except both ends thereof at which said crownings are formed.
 14. A gear shaft support device for a vehicle in which a gear shaft is rotatably supported by tapered roller bearings in a housing in which is sealed gear oil, said tapered roller bearings each having an outer ring, an inner ring, and tapered rollers, characterized in that said inner ring has a large rib surface made up of a conical surface brought into contact with large end faces of said tapered rollers, and a flank smoothly connecting with said conical surface and curving in a direction away from the large end faces of said tapered rollers.
 15. The gear shaft support device as claimed in claim 14 wherein said flank has a circular section.
 16. The gear shaft support device as claimed in claim 15 wherein a circular recess is provided on the central portion of each of the large end faces of said tapered rollers, and the outer peripheral end of said recess extends to near the boundary between said conical surface and said flank of said large rib surface of said inner ring.
 17. The gear shaft support device as claimed in claim 15 wherein the boundary between said conical surface and said flank of said large rib surface of said inner ring is provided near the outer edge of the maximum contact oval produced by the contact between the large end faces of said tapered rollers and the large rib surface of said inner ring under the maximum permissible axial load of said tapered roller bearing.
 18. The gear shaft support device as claimed in claim 14 wherein a circular recess is provided on the central portion of each of the large end faces of said tapered rollers, and the outer peripheral end of said recess extends to near the boundary between said conical surface and said flank of said large rib surface of said inner ring.
 19. The gear shaft support device as claimed in claim 18 wherein the boundary between said conical surface and said flank of said large rib surface of said inner ring is provided near the outer edge of the maximum contact oval produced by the contact between the large end faces of said tapered rollers and the large rib surface of said inner ring under the maximum permissible axial load of said tapered roller bearing.
 20. The gear shaft support device as claimed in claim 14 wherein the boundary between said conical surface and said flank of said large rib surface of said inner ring is provided near the outer edge of the maximum contact oval produced by the contact between the large end faces of said tapered rollers and the large rib surface of said inner ring under the maximum permissible axial load of said tapered roller bearing. 